1. Field of the Invention
The present invention relates to ultrasound imaging systems and, more particularly, to ultrasonic color flow imaging.
2. Description of the Related Art
Ultrasound imaging systems are known to display images associated with blood flow in a subject. Such a display mode is known as a color flow mode because different flow directions are indicated by different colors. Additionally, the velocity of the flows are indicated by degree of brightness of the colors.
Displaying blood flow on a display device is known to be problematic. In particular, with stationary or slow moving blood, it is difficult to display accurate images of blood flow that are not confusing to a clinician. With conventional ultrasound imaging systems, blood flow that is either stationary or slow moving in a particular direction often leads to measurement inaccuracies by the ultrasound imaging system that cause blood flow to appear to be moving when it is stationary or appears to be moving in an opposite direction than it is actually moving. In addition, for the portions of an image being displayed that correspond to stationary or slow moving blood, there tends to be small interspersed areas of blood flowing to the transducer as well as away from the transducer. In a two color display mode, where red is used for blood movement in one direction and blue is used for blood flowing in the opposite direction, a clinician can be confused by the resulting image displayed because red and blue pixels will often be interspersed. Consequently, the clinician may be mislead or be unable to diagnose the subject's ailment.
Attempts to overcome the problems associated accurate display of slow moving blood have been somewhat successful. Generally, the conventional approaches perform some sort of averaging of previous signals with current signals. This averaging approach smoothes out the resulting signals such that the interspersed regions of red and blue pixels are largely prevented. However, other regions not needing the smoothing are also smoothed, and thus distorted. Hence, confusion or misdiagnosis by a clinician can still occur because the high velocity signals are now distorted.
Another problem that occurs due to the averaging of the previous signal and the current signal is that unacceptably long decay times for colors being displayed may occur. As an example, very long decay times may result in data being displayed even after the probe is no longer receiving information. Such long decay times cause the resulting image to lag and often distort actual flow conditions.
Recently, averaging approaches have been enhanced to overcome some of the above-mentioned problems. In U.S. Pat. No. 5,357,580 an improved averaging solution is disclosed in which a variable weighting coefficient is used so as to heavily average image information for smaller velocities and minimally average image information for high velocities. Similarly, U.S. Pat. No. 5,467,770 discloses a filter that provides an averaging solution with persistence to ultrasound images by using filter adjustable weighting coefficients. Further, in U.S. Pat. No. 5,215,094 another similar averaging solution is disclosed. The averaging of a new value with a previous output value is performed by a persistence modulator to provide asymmetric persistence to the image displayed pertaining to fluid velocity. In particular, increases in the velocity are processed to exhibit a shorter time constant than the time constant used for decreases in velocity.
However, these recent averaging approaches still have significant shortcomings. There is no accommodation for velocity values that have rapidly changed direction. In addition, small flow detection is hampered for low flows, particularly when flow direction reverses. Further, when aliasing is present due to high flow velocities, the resulting image is typically not accurately displayed. Also, corrupt velocity values distort actual velocity values in the averaging approach.
Thus, there is a need for an improved approach to imaging velocity of fluids in an ultrasound system such that persistence processing suitably deals with rapid direction changes in flow, low velocity flows, and corrupt signals.